In recent years, research and development of vehicles using ethanol as fuel have been actively carried out. Within a fuel tank storing ethanol fuel, in a case in which the ethanol concentration is substantially 100%, the air/fuel ratio at normal temperature is often in a combustible range, and for a vehicle equipped with this fuel tank, even in the event that a flame comes close to a fuel filler orifice of the fuel tank for some reason, it is desirable that the flame is prevented from entering the fuel tank.
In an automobile, a fuel filler guide tube extending upward from a fuel tank is generally equipped with a device such as a shutter that is normally retained in a closed state and is opened when a fuel filler gun is inserted into the fuel filler guide tube (see Patent Publication 1), and there is therefore no possibility that a flame will enter the fuel tank. However, in a motorcycle such as a motorbike, in terms of layout of a fuel tank, it is not realistic to mount a long fuel filler guide tube on the fuel tank, and it is therefore difficult in practice to provide a shutter, etc. as for an automobile.
Providing a flame arrestor made of a metal mesh within a narrow fuel filler guide tube to prevent a flame from entering the fuel tank can be considered, but in this case consideration must be given to degradation of the fuel filling performance since an oil film is formed in many of the mesh holes of the flame arrestor due to the surface tension of the fuel when it comes into contact with the whole of the flame arrestor when filling with fuel, as a result of which the replacement of air discharged from the fuel tank to the fuel filler guide tube side with fuel flowing from the fuel filler guide tube to the fuel tank side is inhibited in the flame arrestor.    Patent Document 1: Japanese Patent Application Laid-open No. 2001-138755